The present invention relates to semiconductor waveguide devices, and in particular to methods for controlling the optical output beam divergence therefrom.
The invention has particular, though not exclusive, use in the manufacture of semiconductor lasers, amplifiers, modulators and other waveguides suitable for use in telecommunications and printing applications where low coupling loss to other optical components (such as optical waveguides) and high kink-free power output is required. More particularly, the invention has particular, though not exclusive, use in the manufacture of 980 nm pump lasers for telecommunications applications, and in the manufacture of 830 nm high power lasers for printing applications.
Conventional semiconductor lasers generally provide an optical output beam of substantially elliptical cross-section. With reference to FIG. 1, such conventional lasers typically comprise a succession of layers formed on a substrate 10, to include a lower cladding layer 11, an optically active core region 12 and an upper cladding layer 13. In a conventional ridge type laser, lateral optical confinement is effected by way of a ridge waveguide 14 upon which suitable electrical contact material 15 may be deposited with which to provide electrical injection to drive the lasing mode. The resultant optical output beam 16 emerges from the output end 17 along the longitudinal or z-axis as shown.
The core region 12 typically comprises a plurality of layers, such as a central layer 12a defining the quantum well and two or more outer layers 12b, 12c with composition varying as a function of depth within the layer to provide a so-called graded index separate confinement heterostructure (GRINSCH) core region 12.
The structure of FIG. 1 provides a conduction band edge profile as shown in FIG. 2, in which the cladding region 20 corresponds to the lower cladding layer 11; the cladding region 24 corresponds to the upper cladding layer 13; the quantum well region 22 corresponds to the central layer 12a; and the graded index regions 21 and 23 respectively correspond to the outer layers 12b, 12c. 
The output beam 16 (FIG. 1) from this structure typically has a beam divergence in the vertical direction (shown in the figure as the y-direction) of the order of between 30 and 40 degrees, resulting in a large vertical far field. Vertical direction is generally defined as the wafer growth direction, i.e. the direction that is orthogonal to the plane of the substrate, as shown. By contrast, beam divergence in the horizontal or lateral direction (shown in the figure as the x-direction) is typically of the order of between 5 and 10 degrees.
The very large far field and asymmetry of the far fields in the vertical and horizontal directions cause a number of problems such as high coupling loss to optical components downstream of the laser output 17 (such as optical fibres) and particularly low coupling tolerance between the laser and a single mode fibre (which requires a circular beam profile).
A number of techniques have been proposed to reduce the far field or beam divergence in the vertical direction and hence to reduce the asymmetry in far field output. The vertical far field can be reduced to some degree by simply reducing the thickness of the core region 12. However, in this case, optical overlap with the quantum well 12a is also reduced which in turn increases the laser threshold current and cavity loss occurs from losses associated with intervalence band absorption. In addition, it becomes more difficult to achieve high kink-free emission power due to the occurrence of higher mode lasing at high current injection levels.
A number of different approaches have been proposed to provide ‘mode-shaping’ layers that reduce far field into the semiconductor laser structure.
For example, U.S. Pat. No. 5,815,521 describes a laser device in which a mode-shaping layer is introduced into each one of the cladding layers. Each of the mode-shaping layers comprises a layer of increased refractive index relative to the rest of the respective cladding layer, to form a conduction band edge profile as shown in FIG. 1 of US '521. It is noted that the mode-shaping layers each comprise a localized step change in band energy level.
U.S. Pat. No. 5,923,689 describes another technique in which a pair of passive waveguides of reduced refractive index is provided on either side of the quantum well structure, separated therefrom by a barrier layer. A similar structure, this time also in conjunction with a graded index confinement structure, is also shown in “Semiconductor lasers with unconventional cladding structures for small beam divergence and low threshold current” by Shun-Tung Yen et al, Optical and Quantum Electronics 28 (1996) pp. 1229-1238. Both of these documents advocate a step-wise local reduction in refractive index within the cladding layers.
“Design and fabrication of 980 nm InGaAs/AlGaAs quantum well lasers with low beam divergence” by Guowen Yang et al, SPIE Vol. 2886 (1996), pp. 258-263 describes the insertion of two low refractive index layers inserted between the cladding and graded index layers to decrease beam divergence in the vertical direction.
“980 nm InGaAs/AlGaAs quantum well lasers with extremely low beam divergence” by Shun-Tung Yen et al, Proceedings of Semiconductor Laser Conference (1996), 15th IEEE International, pp. 13-14 also describes the introduction of two low refractive index layers respectively adjacent to the graded index layers also to reduce the vertical far field distribution.
U.S. Pat. No. 6,141,363 contemplates the reduction of beam divergence by means of a plurality of layers within the core region, of alternating high and low refractive index.